1. Field of the Invention
The present invention is directed to a device for the insertion of local gradient colls into a magnetic resonance apparatus.
2. Description of the Prior Art
Magnetic resonance systems have been employed for many years in medicine and in biophysics for acquiring images from the inside of the body of an examination subject. Magnetic resonance tomography is based on the physical phenomenon of nuclear magnetic resonance. In the examination procedure, the examination subject is exposed to a strong, constant magnetic field, causing the previously irregularly oriented nuclear spins of the atoms to align in the examination subject. These unordered nuclear spins are excited to a specific oscillations (resonant frequency) by means of radio-frequency waves. This oscillation generates the actual measured signal (RF response signal) for the actual image acquisition in magnetic resonance tomography that is picked up by suitable reception coils.
A pre-condition for the image acquisition is an exact identification of the respective locations at which the Rf response signals arise in the examination subject (location information, or location encoding). This location information is acquired by means of auxiliary magnetic fields (magnetic gradient fields) that are generated along the three spatial directions by gradient coils. The gradient field superimposed on the basic field is designed such that the field strength, and thus the resonant frequency as well is different in every volume element. When RF energy at a defined resonant frequency is emitted into the subject, only those atoms can be excited that are situated at a location at which the magnetic fields satisfy the corresponding resonance condition. Suitable changes of the gradient fields make it possible to shift the location of such a volume element at which the resonance condition is satisfied in a defined way, and to thus scan the desired region.
For the examination, the subject is introduced into the examination space of the magnetic resonance apparatus and positioned therein. In order to be able to accept examination subjects of different sizes, the examination space must have a minimum size. For medical applications, the size is selected such that a patient can be completely introduced into the examination space.
A local gradient coil unit (insert gradient coil) can be employed for the examination of specific regions of an examination subject, for example the head of a patient, U.S. Pat. No. 5,185,576 discloses such a local gradient coil unit that is introduced into the examination space of the magnetic resonance apparatus.
With their smaller dimensions, local gradient coils have advantages over the built-in whole-body gradient coils of a magnetic resonance apparatus in terms of the obtainable gradient intensity and the power demands made on the gradient amplifier that feeds the gradient coil unit. For operation, the local gradient coil unit together with the integrated, local radio-frequency antenna must be firmly anchored in the examination space of the magnetic resonance apparatus in order to withstand the forces acting on them.
In some embodiments of local gradient coils the securing of the local radio-frequency antenna in the magnetic resonance apparatus and the removal therefrom can require several hours, since the whole-body antenna of the magnetic resonance apparatus must be un-installed and re-installed in some cases.
U.S. Pat. No. 5,311,134 discloses a magnetic resonance apparatus that has a rail-like guide mechanism on which a movable gradient coil unit can be moved. The magnetic resonance apparatus further has a patient bed with which a patient lying thereon can be positioned both in the interior hollow volume of the examination space as well as in the interior hollow volume of the local gradient coil unit.
U.S. Pat. No. 5,372,137 discloses an arrangement for the insertion of a local gradient coil into the examination space of a magnetic resonance apparatus over the patient bed.
A local gradient coil unit seated on the patient bed that is movable relative to a patient borne on the patient bead is disclosed by U.S. Pat. No. 5,197,474.
U.S. Pat. No. 4,567,894 A discloses a patient bed that can be coupled to and uncoupled from a magnetic resonance apparatus by means of a coupling mechanism.
Heretofore, the insertion and mounting of the local gradient coils, which can have a weight of up to 250 kg, has only been manually possible and requires a high outlay of the operating personnel in terms of time and extension.